1. Field of the Invention
The present invention relates to a radar, and particularly relates to an FMCW radar.
2. Description of the Related Art
A safety of automobile has been regarded as important in recent years, and an on-vehicle millimetric wave radar for avoiding collision and the like, for instance, has been prospected to be used broadly as one of the techniques for improving the safety. For a radar to be used as the on-vehicle radar or the like, an FMCW radar can be cited.
The FMCW radar is a radar which FM-modulates a carrier wave to make it a transmitting signal, and performs reception by using a signal being a branched transmitting signal as a local signal for down-conversion at the time of reception. When the transmitting signal from the radar hits an object and the radar receives the reflected signal, a beat with a frequency in proportion to a delay time in accordance with a distance to the object can be obtained. The FMCW radar measures the distance and a relative speed to the object using the beat frequency.
Incidentally, in a device using a semiconductor element, there exists a noise called a flicker noise with low frequency. This is a noise caused by a fluctuation of carrier or the like due to an impurity on a crystal interface inside the semiconductor. Normally, the flicker noise becomes more dominant than a white thermal noise at about 1 MHz or under (hereinafter, a frequency at which the flicker noise generally becomes more dominant than the white noise is called “cut-off frequency of flicker noise”), and is also called as 1/f noise because of its frequency characteristic. When a close distance is detected in the FMCW radar, since the beat frequency is generated at a relatively low frequency, there is a problem that an S/N ratio (SNR) is deteriorated due to the flicker noise.
As a method of preventing the deterioration of SNR due to the flicker noise, there is known a technique in which a flicker noise generated by a mixer at a high frequency side is avoided by performing down-conversion in two times (refer to JP-A 10-319113 (KOKAI)).